A "Triad" is a chord with 3 notes consisting of the
Root [1st note]
So if we take a major scale like C major, we'll build a chord with "C" as the root:
C D E F G A B C
C is the 1st note [Root]
E is the 3rd note &
G is the 5th note
and that's it, a simple 3 note chord, the C major triad, using notes C, E & G
Have a look below at the C major chord.
It consists of the notes C, E & G
So across the 6 strings of the guitar you play an arrangement of those three notes only!
Using the C major scale you could construct another chord say starting on the "F" note.
"F"would now be the 1st note, "A" would be the 3rd & "C" would be the 5th
You have now built the chord of "F Major" with notes F, A & C !